Sequential administration of neuropsychological tests is an integral component of longitudinal study of Alzheimer's disease, but short-term variability has not been delineated for most existing measures. Short-term variability will be evaluated through test-retest administration of standardized tests of cognition and behavior to a cohort of 110 persons with clinically-diagnosed Alzheimer's disease and 110 persons without neuropsychiatrist abnormality. The descriptive aim of this study will be to characterize test-retest changes in summary scores of measures of cognition and behavioral disturbances. In slow developmental (e.g., aging) and degenerative (e.g., Alzheimer's disease) processes, short-term variability should primarily reflect random error. This study will also test the independence of this error component from several potential sources of variance. For cognitive measures, these sources of variance include: initial test score, global dementia severity, age, years of formal education, and gender. The same model terms will also be tested for behavioral measures with the addition of descriptive variables pertaining to the informant, the source of behavioral data. Immediate applications of study results will be: the design of longitudinal studies of the progression of cognitive and behavioral disturbances in Alzheimer's disease, and the design of clinical trials, with particular focus on remediable behavioral symptomatology. Item response patterns within measures generates valuable information for future scale development.